1. Field of the Invention
The invention relates to covalent modification of proteins through their conjugation with other proteins. More particularly, the invention relates to the modulation of such conjugation involving the protein NEDD8.
2. Summary of the Related Art
Covalent modification of proteins through their conjugation with other proteins is an important biological mechanism for regulating protein metabolism and biological activity. Hershko and Ciechanover, Annu. Rev. Biochem. 61: 761-807 (1992) discloses conjugation of ubiquitin, one of the most conserved eukaryotic proteins, to other proteins through an enzymatic mechanism, as well as its role in protein degradation. Rock et al., Cell 78: 761-771 (1994) discloses that ubiquitination of protein antigens is required for processing of such antigens. Murray, Cell 81: 149-152 (1995), teaches that ubiquitination of cyclin is involved in cell cycle regulation. Scheffner et al., Cell 75: 495-505 (1993) discloses that ubiquitination of p53 is involved in degradation of this tumor suppressor.
The enzymatic pathway for ubiquitination has been reasonably well defined. Jentsch, Annu. Rev. Genet. 26: 179-207 (1992) discloses that ubiquitination requires initial activation of a conserved C-terminal glycine residue by the ubiquitin activating enzyme, E1, through formation of ubiquitin adenylate in an ATP-dependent process which liberates PPi, followed by thiol ester formation at a thiol site in E1 with release of AMP. Ubiquitin is then transferred to a thiol site in ubiquitin conjugating enzyme, E2, through formation of a thiol ester bond. Ubiquitin is then transferred to an epsilon amino group of a lysine residue in the target protein through an amide linkage, usually with the involvement of ubiquitin-protein isopeptide ligase, E3. Hopkin, J. Natl. Inst. Health Res. 9: 36-42 (1997), teaches that target specificity is regulated by the particular combination of E2 and E3 protein, with more than 30 E2 proteins and 10 E3 proteins being known at present.
Ubiquitin is not the only protein which is used to modify other proteins through covalent linkage, however. Kamitani et al., J. Biol. Chem. 272: 14001-14004 (1997), discloses that sentrin, a ubiquitin-like protein, appears to be processed similarly to ubiquitin, but has a smaller target protein repertoire than ubiquitin. Okura et al., J. Immunol. 272: 4277-4281 (1996) teaches that sentrin protects cells against anti-FAS and tumor necrosis factor-mediated cell death. Loeb and Haas, J. Biol. Chem. 267: 7806-7813 (1992), discloses that ubiquitin cross-reactive protein (UCRP), which contains two ubiquitin domains, is conjugated to a large number of intracellular proteins. Kumar et al., Biochem. Biophys. Res. Commun. 185: 1155-1161 (1992), discloses another ubiquitin-like protein, called NEDD8, for Neural precursor cell-Expressed Developmentally Down regulated. Kamitani et al., J. Biol. Chem. 272: 28557-28562 (1997), teaches that NEDD8 is predominantly expressed in the nucleus and is conjugated to target proteins through a mechanism analogous to ubiquitination.
These proteins, which covalently modify other cellular proteins, are important components of biological regulatory processes. The nuclear expression pattern and developmental regulation of NEDD8 make it a particularly compelling candidate as an important regulatory molecule. There is a need, therefore to understand the role of NEDD8 in biological regulation. Unfortunately, the lack of understanding about the specific proteins involved in NEDD8 conjugation has resulted in a lack of effective tools to probe the role of NEDD8. There is, therefore, a need for better tools to utilize in elucidating the role of NEDD8 in biological regulation. Ideally, such tools would allow modulation of the activation and/or conjugation of NEDD8.